The present invention relates generally to a head drum for a video tape recorder. More particularly, the invention relates to an improved head drum which forms a uniform air film between a recording medium such as a magnetic tape and an area of the head drum wrapped by the tape to reduce wear on the tape and on the peripheral surface of the head drum and to reduce resistance to tape travel.
The recording operation of, for example, a video tape recorder (VTR) or a rotary head type digital audio tape recorder (R-DAT) is carried out by a head drum which forms tracks on a magnetic tape wrapped about the head drum, extending at an angle with respect to the direction of tape travel. The conventional head drum typically includes a stationary drum installed on a mechanical chassis, a rotary drum rotatable relative to the stationary drum spaced with a predetermined axial gap from the stationary drum, and a plurality of magnetic heads exposed at an outer surface of the rotary drum. A lead surface having a stage edge is formed to extend helically on the peripheral surface of the head drum. When recording or reproducing, the magnetic tape travels along the lead surface while wrapping thereabout at a given angle according to the rotation of the head drum. Diagonal recording tracks are formed on the magnetic tape by the head drum in the recording operation, while in a play operation the head drum scans the diagonal tracks to read signals from the magnetic tape.
Rotation of the head drum causes circumambient air to follow the peripheral surface thereof. The air flows between the magnetic tape and the peripheral surface of the head drum from the position where the magnetic tape starts to wrap about the head drum forming an air film. Accordingly, the tape wrapped about the head drum travels thereover assisted by the air film.
It will be appreciated that, as the magnetic tape travels, it is separated slightly from the peripheral surface by the air film, preventing wear on the outer surface of both the head drum and the magnetic tape and reducing tape traveling resistance to achieve stable operation.
However, the formation of the air film is dependent upon the direction of magnetic tape travel and the rotation of the rotary drum, often resulting in an uneven air film thickness. Usually, the thickness of the air film is greater at the initial tape wrap position, while it becomes thinner approaching the final wrap position. Thus, the separation of the magnetic tape from the peripheral surface of the head drum is too great at the initial position for stably engaging the rotary head thereat and is too close to the rotary drum at the final wrap position. Due to this variation in the engagement of the magnetic tape with the head drum from the initial tape wrap position to the final wrap position, the frequency characteristics produced when recording or reproducing are diminished and tape traveling resistance is increased, resulting in damage to the magnetic tape, wear on the head surfaces, and playback jitter.
To solve such problems caused by the uneven air film, it has been proposed that the back tension of the magnetic tape be adjusted, and that a hole be provided to communicate with the inside of the head drum and formed near the initial tape wrap position to partially discharge air to the inside thereof so as to maintain the air film at a constant thickness in the vicinity of the initial tape wrap position, together with the provision of a pump-out groove formed between the upper and lower drums for positively discharging air from the inside of the head drum to the outside thereof.
However, it is practically impossible to form a uniform air film by adjusting the back tension in order to vary the tension between the initial and final tape. Moreover, the presence of the hole causes injury to the magnetic tape at the edge of the hole. Further, with the use of the pump-out groove, the air film thickness is increased at the final tape wrap position, but the air film becomes too thick in the vicinity of the initial tape wrap position. It has been proposed to select the size or orientation of the groove to form a uniform air film. This means however cannot sufficiently achieve a uniform air film over the entire range between the initial and final tape wrap positions.